Simple Code to add a form on a webpage when code entered

I want users to go to my homepage: <[login to view URL]>? Then enter a discount code, which will open? up a form so they can join for free. I have done an extra website to show you what I want it too look like after the? code is? applied: <[login to view URL]> ? ## Deliverables I want users to go to my homepage: [[login to view URL]][1]? Then enter a discount code, which will open? up a form so they can join for free. I have done an extra website to show you what I want it too look like after the? code is? applied: <[login to view URL]> More Detailed: I will want to use multiple codes, this do not have to be secure or anything. The easier the better. (Can do it so any code works, if its easier). If the exact solution shown above cannot be done, as long as nearly the same can be done, this might work. The overall idea is that potential customers enter a discount code that I have given them making an email capture form? (along with some text) appear on the same page so that they can easily sign up.